Energy budgets of windbreak canopies in the Sahel
Windbreaks are openly exposed to turbulent motions in the atmosphere because they are taller than adjacent crops and usually only one or two trees in width. Transpiration by trees in windbreaks, consequently, can be expected to deviate substantially from the equilibrium rate as a result of advection and vertical entrainment, which may therefore influence water budgets for windbreak systems and competitive interactions between trees and crops. To quantify the effects of advection and entrainment on transpiration by windbreak trees, the energy budgets of canopies of Azadirachta indica A. Juss. windbreaks at a site in southern Niger were measured at four different times of year. Each component of the energy budget was determined independently and expressed per unit of windbreak length: net radiation (Rn) was measured using an array of eight linear net radiometers enclosing the windbreak foliage; latent heat fluxes (λAE) were determined by scaling sap flow, measured using the heat-pulse technique, from tree to windbreak length on the basis of leaf area; sensible heat fluxes (H) were determined from measurements of the difference in temperature between the crown of a windbreak tree and ambient air, using aerodynamic conductances estimated by scaling-up in situ measurements of leaf boundary layer conductances; the change in heat storage by the canopy (ΔS) was estimated from changes in wood temperature over time. ΔS was found to be a negligible component of the energy budget of windbreak canopies and comparison of λE + H with Rn showed good agreement. Rn was mostly dissipated as λE during the rainy season and early dry season but as H during the driest period of the year. Examination of exchange fluxes for the windbreaks showed that partitioning of energy by windbreak canopies was strongly affected at all times of year by exchanges of energy between sensible and latent heat at canopy surfaces as a result of advection and entrainment. During moist periods of the year, advection and entrainment enhanced λE relative to λEeq the latent heat flux at equilibrium, by up to 60%, but depressed λE during the driest time of year, so that λE was as low as 40% of ΛEeq Depression of ]glE relative to λEeq by advection and entrainment occurred when surface conductances of the windbreaks were reduced by lack of moisture, whereas λE was enhanced when surface conductances were high. The effects of advection and entrainment on water use by windbreak trees must be accounted for when assessing the wager budgets of windbreak systems, because any reductions in evapotranspiration by crops resulting from shelter may be offset by enhancement of tree transpiration.
Agricultural and Forest Meteorology (1996) 86 (1-2) 33-49 [doi:10.1016/S0168-1923(96)02416-1]